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Author Topic: Not really about "The Noise"  (Read 1890 times)

BendingSince1997

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Not really about "The Noise"
« on: March 04, 2015, 02:07:25 PM »

I'm building something where in use a keyboard motherboard and a 555 timer.. ECT..  Point of the story is i'm experimenting with a power source for the 555 timer so it doesn't affect the output of the keyboard. I took the motherboard from an old broken tablet and took the two wires that go to the led display in the screen. when the "led display is dim" and i have an LED on my bread board the LED is dim. But if i press a button on the tablet motherboard and the "led displays brightness is active" the LED is not only bright but it begins to scream a high pitch.
On another note i have recently added another 555 timer for other reasons and all the negative is powered through the same line for all the ICs and it will continuously loop on the new 555 timer even though there is no positive going to said 555 timer and I've pulled the jumper connecting to said 555 timer and the loop stops but when i plug it back in it starts again.
I'm not sure why its looping without the positive power when i have it set up the same way with Diodes going to the keyboard so information isn't sent back unless its let through the 555 timer.
If anyone could help me with any of these problems that will be great ill be here to reply if needed.
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gert

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Re: Not really about "The Noise"
« Reply #1 on: March 07, 2015, 02:53:47 AM »

Get a proper PSU, preferably real transformers or battery, build a 4 diode rectifier and proper caps on secondary, then use classic 7805, 7809 whatever to generate the needed voltages... screen lights are dimmed by PWM, so that is your buzz. The other noise may be a ground loop. Make star shaped grounding, center point should always be near the PSU. Try to split digital and audio ground, though I do not really see through what you do with the 555s and how you connect a keyboard there. If you could share a scribbling of your circuit it can maybe be said more precise.
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